1. Field of the Invention
The present invention relates to a developing apparatus used with an electrophotographic apparatus, an electrostatic recording apparatus or the like.
2. Related Background Art
Conventionally, various electrophotographic methods have been proposed, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 and Japanese Patent Publication No. 43-24748. However, in general, there has been used a method in which an electrostatic latent image is formed on a photosensitive member by means of various means utilizing photo-conductive material and the electrostatic latent image is developed and visualized as a toner image by using toner and after the toner image is transferred onto a transfer material such as a paper sheet the toner image is fixed to the transfer material by heating or solvent vapor, thereby obtaining a copy.
Further, various methods for visualizing the electrostatic latent image by using the toner are already known. For example, there have been proposed various developing method such as a fur brush developing method, a cascade developing method and a liquid developing method, as well as a magnet brush developing method as disclosed in U.S. Pat. No. 2,874,063 and a powder cloud developing method.
Among these developing methods, particularly, the cascade developing method and the magnetic brush developing method using toner and carrier mainly have widely been used. These methods are excellent developing methods capable of providing a high quality image relatively stably.
However, all of the above-mentioned developing methods have common disadvantages such as degradation of the carrier, change in mixing ratio between the toner and the carrier, complexity of the apparatus, scattering of the toner and unevenness stripes due to existence of the carrier, which are inherent to two-component developing agent.
To eliminate the above-mentioned disadvantages, various developing methods utilizing one-component developer consisting of toner have been proposed. For example, U.S. Pat. No. 3,909,258 discloses a developing method using electrically conductive magnetic toner in which the conductive magnetic developer is born on a cylindrical conductive sleeve having a magnet therein and an electrostatic latent image is developed by contacting the sleeve with the latent image. In this case, in a developing portion, toner particles forms a conductive path between a surface of a recording body (photosensitive member) and a surface of the sleeve, and charges are supplied to the toner particles from the sleeve through the conductive path, and the toner particles are adhered to the electrostatic latent image on the recording body by a Coulomb force between the latent image and the toner particles, thereby effecting development. This developing method using the conductive magnetic toner is excellent in the point that the disadvantages of the conventional two-component developing methods can be eliminated, but, has a disadvantage that, since the toner is conductive, it is difficult to electrostatically transfer the developed image from the recording body to a final support member such as a normal paper sheet.
To eliminate this disadvantage, as a developing method using high resistance toner capable of being transferred electrically, Japanese Patent Application Laid-Open No. 52-94140 discloses a developing method utilizing dielectric polarization of the toner particles.
However, this developing method has disadvantages that a developing speed is inherently slow and that density of the developed image is insufficient, and, thus, is hard to be practically used.
As another developing method using the high resistance magnetic toner, a developing method in which toner particles are frictionally charged by friction between the toner particles and friction between the toner particles and a sleeve and the toner particles are contacted with an electrostatic holding member to achieve development has been proposed.
However, this method has disadvantages that poor frictional charging is apt to occur since the number of contacts between the toner particles and the friction member is small and that, if the Coulomb force between the charged toner particles and the sleeve is strong, the toner particles are apt to be aggregated on the sleeve, and, thus, it has been pointed out that this method is hard to be practically used.
To eliminate these disadvantages, a new developing method capable of eliminating the above-mentioned disadvantages has been proposed, as disclosed in Japanese Patent Application Laid-Open No. 54-43036. In this method, after a very thin toner layer is formed on a sleeve by coating and the toner layer is frictionally charged, the toner layer is closely approached to an electrostatic latent image within a magnetic field, thereby achieving development without contact between the toner and the latent image. According to this method, since the magnetic toner is coated as the very thin layer, the chance of contact between the magnetic toner particles and the sleeve is increased, with the result that frictional charges required for development can be applied to the toner particles.
As a result that inventor tested and checked the application of charges in the above-mentioned one component developing method, it was found that movement of the toner particles at the charge applying portion in the one-component developing method becomes as follows.
FIG. 5 shows an example of a developing apparatus using one-component magnetic toner.
Namely, FIG. 5 is a sectional view of a developing apparatus 3 in which a developing sleeve 1a formed from non-magnetic material can be rotated in a direction shown by the arrow. A permanent magnet 1b is fixed within the developing sleeve 1a. The developing apparatus further includes a magnetic blade 2 made of magnetic material, and a convey member 4. The magnetic blade 2 is disposed in such a manner that a distance W between the magnetic blade and the developing sleeve 1a becomes constant. Incidentally, in general, in most cases, the distance W is selected to 100 .mu.m to 1 mm.
In the developing apparatus 3 shown in FIG. 5, one-component magnetic toner T is coated on the developing sleeve 1a as a thin toner layer having a thickness which is determined by a position of a cut line L shown in FIG. 7.
According to the inventor's investigation, it was found that the charges are applied to the magnetic toner when the magnetic toner T passes through a space between the developing sleeve 1a and the magnetic blade 2. Further, it was found that the movement of the magnetic toner T in this case becomes as follows.
As shown in FIG. 6, among planes perpendicular to a line connecting between the developing sleeve 1a and the magnetic blade 2, when it is assumed that the plane nearer to the magnetic blade 2 is S1 and the plane nearer to the developing sleeve 1a is S2, generally, since a width of the magnetic blade 2 is smaller than a width of the permanent magnet 1b, magnetic flux density on the plane S1 becomes greater than magnetic flux density on the plane S2. Accordingly, the magnetic toner T is subjected to a force directing toward the magnetic blade 2 (shown by the arrows in FIG. 6) between the developing sleeve 1a and the magnetic blade 2.
Thus, as shown in FIG. 6, the magnetic toner T forms "ears" (condition shown in FIG. 7) which extend from the magnetic blade 2 to the developing sleeve 1a. The application of charges to the magnetic toner T is achieved by increasing density of toner at the ears and by contacting the developing sleeve 1a and toner particles t1 on the tip ends of the ears formed from the magnetic blade 2 to apply charges to the toner particles.
Further, it was found that conveyance of toner between the developing sleeve 1a and the magnetic blade 2 is effected as follows.
As mentioned above, since the charges are applied to the toner particles t1 (on the tip ends of the ears) contacted with the developing sleeve 1a, the toner particles are subjected to a force (generated by a mirror symmetry force (mirroring force)) directing toward the developing sleeve 1a and are also subjected to a conveying force (due to a frictional force between the toner particles and the developing sleeve 1a) directing toward a rotational direction of the developing sleeve 1a.
Further, since there is a certain cohesive force between the toner particles, toner particles t2 contacted with the toner particles t1 are also subjected to the conveying force due to the presence of the cohesive forces. Similarly, toner particles t3 (on the upper surface of the toner layer) are also subjected to the conveying force due to the presence of the cohesive forces.
However, as mentioned above, since there is the magnetic force directing toward the magnetic blade 2 between the developing sleeve 1a and the magnetic blade 2, the toner ears are cut at a location where the conveying force overcomes the magnetic force (i.e., on the cut line L shown in FIG. 7), with the result that only the toner particles remaining on the developing sleeve 1a are conveyed toward the rotational direction of the developing sleeve 1a.
Accordingly, in systems wherein the magnetic toner cohesive force is great or in systems wherein magnetic toner having the greater number of contacts for obtaining the required amounts of frictional charges is used, toner particles not contacted with the developing sleeve and having poor charges are conveyed to a developing area, thereby creating a poor image due to poor charging.
To avoid this inconvenience, the inventor has proposed a developing apparatus 3 (as shown in FIG. 8) comprising developer including magnetic toner, a rotatable developing sleeve (developer bearing member) 1a for bearing the magnetic toner on a surface thereof, a permanent magnet 1b disposed within the developing sleeve 1a, and a developer regulating member 6a for regulating a coated amount of the magnetic toner, wherein at least two conveying forces directing in opposite directions are applied to the magnetic toner at a developer regulating portion, and, at least one of the conveying forces is a force for conveying the toner toward a developing area and mainly depends upon a charged amount of the toner, and, the other conveying force (acting toward a direction opposite to the developing area) depends upon a rotational force and a magnetic force acting from the developer regulating member 6a to the magnetic toner, so that non-charged magnetic toner is not subjected to the conveying force directing toward the developing area. As a result, only the sufficiently charged magnetic toner is coated on the surface of the developing sleeve 1a uniformly, with the result that only the sufficiently charged magnetic toner can be conveyed to the developing area. Incidentally, in FIG. 8, the reference numeral 9 denotes a photosensitive drum as an electrostatic latent image bearing member.
In association with such a developing device 3, the inventor has proposed a magnetic seal member 7 (as shown in FIG. 9) made of magnetic material and adapted to cover peripheries of the developing sleeve 1a and the developer regulating member 6a in a "3"-shaped (inverted E shaped) manner, as an end toner leakage preventing member of non-contact type.
At an edge portion A of the developer regulating member 6a side shown in FIG. 8, the magnetic seal member 7 shown in FIG. 9 serves to coat the magnetic toner on the developer regulating member 6a at an area having a width equal to a width of the magnetic seal member 7 in the same manner that the toner is coated by the magnetic blade 2 at a magnetic pole portion N61 within the developer regulating member 6a. The coated magnetic toner is returned to a developing container while passing through a space or gap between the developing sleeve 1a and the developer regulating member 6a and is restrained again at an edge portion B of the magnetic seal member 7 in the space between the developing sleeve 1a and the developer regulating member 6a, thereby preventing toner leakage at the edge portion of the developer regulating member 6a.
Further, regarding the developing sleeve 1a side, the edge portion B of the magnetic seal member 7 in the space between the developing sleeve 1a and the developer regulating member 6a, a phenomenon similar to the toner coating of the magnetic blade 2 is generated so that the toner is coated on the developing sleeve 1a, and the coated toner is restrained again at an edge portion C of the magnetic seal member 7, thereby preventing toner leakage at the edge portion of the developing sleeve 1a.
However, in the developing apparatus 3 using the magnetic seal member 7 as the end toner leakage preventing member, if an amount of magnetized toner is small or if the cohesive force between the toner particles is great, a width of the toner coated on the developer regulating member 6a by the magnetic seal member 7 becomes greater than the width of the magnetic seal member 7, with the result that the toner coated on the developer regulating member 6a and to be returned to the developing container leaks laterally toward ends of the developing sleeve 1a and the developer regulating member 6a at an edge portion of the magnetic seal member 7. Consequently, the toner may leak at end areas of the developing sleeve 1a and the developer regulating member 6a where the permanent magnets 1b, 6b are not located.